Welding Technologies

Of all assembly technologies for thermoplastic composites, welding – or fusion bonding – seems the most obvious. Welding does not require the tedious surface preparation needed for adhesive bonding nor does it suffer from the stress concentrations seen in (drilled and) fastened composite materials. Several welding technologies are available, mainly differing in the way heat is supplied to the weld interface.

Background/Introduction

Several fusion bonding technologies have been developed for thermoplastic composites, all differing in the way that heat and pressure are applied to the interface. So far, the development of new processes has, however, not been accompanied by the definition of specific testing methods to robustly assess (and compare) the mechanical performance of the thermoplastic composite joints.

The strength of fusion bonded joints is usually characterized through single lap shear testing. Single lap shear coupons are easy to manufacture and the testing is fast and straightforward. The test results are highly dependent on the geometry of the specimen, due to complex stress distribution throughout the specimen, leading to an apparent property and not the desired material/structural property. Some research efforts have been exploring the applicability of other testing techniques such as the double lap shear, short beam strength, double cantilever beam or end notched flexural test to analyze fusion bonded joints in thermoplastic composites.

However, proper selection of a testing technique requires a good understanding of all factors affecting the measured property. Apart from intimate contact development and polymer diffusion, these factors may include interface architecture, polymer crystallinity and surface pre-treatment methods. The important factors governing the measured results will be identified through a combination of comprehensive analytical and experimental work.

Approach

Induction welding, ultrasonic welding, resistance welding, laser-assisted fiber placement, and (autoclave) co-consolidation, are the most widely used fusion bonding techniques in the industry. All these technologies, available at TPRC, are used to manufacture test coupons. The variety in technologies provide means to manufacture specimens with varying properties on the microscale, such as crystallinity or fiber/matrix distribution at the joint interface, which all affect the mechanical performance of the joint. The influence of these factors on the mechanical performance is evaluated through different testing techniques. Two groups of testing techniques are investigated.

The first group encompasses fracture toughness based tests, such as the double cantilever beam or end load split test, while the second group involves strength based tests, such as the single lap shear or short beam shear test. Finally, the microstructural mechanisms governing the output of the different testing techniques are identified by means of experiments and extensive analytical study.

TPRC, the ThermoPlastic composites Research Center in the Netherlands, is an open innovation, research- and development center that aims for thermoplastic composites for a broad range of end use markets.